The present invention relates primarily to the field of methods for filling a cavity with an incompressible fluid. While the invention may be utilized for filling any cavity with an incompressible fluid, it is particularly useful for filling a pressure sensor internal cavity with oil.
Some prior pressure sensors have a pressure sensing element in a sealed internal cavity which is filled with an incompressible fluid, such as silicone oil, rather than having the internal cavity filled with air or some other gas. Since typically the internal pressure sensor cavity is small, it may be difficult to completely fill the internal cavity. In addition, it is typically necessary to insure that no air bubbles or other contaminants are contained in the oil in the internal cavity to insure the accuracy of such prior pressure sensors.
U.S. Pat. Nos. 4,732,042 and 4,686,764 and U.S. Statutory Invention Registration H606 all illustrate prior pressure sensors which have a sealed internal sensor cavity filled with an incompressible fluid/gel. Typically, these assemblies are manufactured by a batch processing method comprising evacuating the internal cavities of many different sensors, then providing an incompressible fluid (oil) in or adjacent the internal cavities typically by immersing all of the sensors in a reservoir of incompressible fluid, and then releasing the vacuum such that the incompressible fluid will totally fill the internal cavity. Then, the internal cavity is sealed by some appropriate technique. U.S. Pat. No. 4,686,764 illustrates a filled pressure sensor in which the sealing technique comprises rolling a membrane over an over-filled internal cavity so as to insure total filling of the internal cavity.
All of the prior techniques which utilize immersing the sensor, either totally or partially, in a reservoir of incompressible fluid involve substantial cleanup process steps thereafter since excess fluid should typically be removed from the surfaces of the sensor which do not form the boundaries of the internal cavity. In addition, the batch processing involved in all the prior techniques which use a common reservoir of incompressible fluid for a plurality of sensors, or which simultaneously batch process a large number of sensors in one vacuum chamber, also require a substantial lengthy period for maintaining a vacuum to insure that the vacuum will remove water/moisture and gas pockets (bubbles) contained in the incompressible fluid itself. This can take an appreciatable time since either a large quantity of incompressible fluid is required to immerse the plurality of sensors in the fluid and/or since the entire volume of a vacuum chamber containing the fluid and a plurality of sensors must be evacuated to implement a batch processing step. Also, prior sealing techniques for the internal cavity, even those that create a seal by force fitting a sealing ball, either may reintroduce air bubbles and contaminants into the fluid, or may be complex and difficult to implement and are therefore not desired.